0001204: Small tweak to MTM loader to ignore some unsupported commands

Description

MTM's effect commands are almost identical to MOD, but in the extended section (0x0E) there are a few key omissions and changes. The one I'm most concerned about is E5y which appears to be very different between MTM and MOD, and does cause trouble playing back some MTMs. Example song in the Steps to reproduce, and I've attached my diff file that I wrote against the soundlib/Load_mtm.cpp file of libopenmpt 0.4.3+r11358

Ugh, the comments for extended effects 6 and 7 should say E6x and E7x, lousy cut and paste on my part! But the descriptions in the double quotes for each extended effect is the exact text I found in the help page in the multitrack module editor itself.

Your better fix looks solid. I felt a bit bad about deciding with ignoring finetune entirely but figuring out the proper fix was a bit above my abilities, so thanks for taking a hard look at this and reworking it! I've tested your patch on the problematic song and it sounds good.